When a bias voltage applies to liquid crystal cell in a long term, the liquid crystal cell may produce residual charge. The accumulated residual charge increases with time, thus, the rotation of liquid crystal may be affected and eventually image sticking occur. To avoid image sticking, conventionally polarity inversion is adopted for preventing the accumulated residual charge from occurring in the liquid crystal cell. In polarity inversion, the common voltage is used as a reference voltage level. Specifically, the voltage level of display data is referred as having a positive polarity when the voltage level is higher than the voltage level of the common voltage; reversely, the voltage level of display data is referred as having a negative polarity when the voltage level is lower than the voltage level of the common voltage. Ideally, the voltage level of the common voltage is configured at the intermediate value between the positive polarity and the negative polarity. However, because the feed-through effect, the voltage levels of storage capacitor and liquid crystal capacitor may be affected by a change of voltage of gate control signal through a couple of parasitic capacitance. Consequentially, the voltage level of common voltage may not ideally locate at the intermediate value between the positive polarity and the negative polarity, which may result image flicker. In addition, because the different manufacturing plants, different batches or even difference between each two frames, the same design parameters may not apply to all products. In order to solve the problem of common voltage offset, conventionally the offset of common voltage is adjusted manually in the production process. However, because the manual adjustment of common voltage is performed in vision manner, manpower is consuming and image quality may vary with the detectors. Moreover, the offset of common voltage may still occur when the parameters or voltage levels of related component change due to the liquid crystal display device has a long-term usage. Today, the offset of common voltage can be adjusted by specific digital system. However, compared with the conventional liquid crystal display device, the liquid crystal display device adopting digital system for the adjustment of the offset of common voltage may need extra hardware components and cost due to the analog-to-digital conversion of voltage difference and the computation of the adjusted common voltage.